They get every bit of D3 from their food and then some. I dont even keep any reptiles under UVB lighting, yet I breed beardies also, and Ive currently got a hatchling beardie thats 12 inches long at less than 2 months old. Ive got a few monitors that dont have any UVB lighting and they do great.
Back to living conditions. A bosc monitor (savannah monitor to the American pet industry) is a medium sized monitor that averages anywhere from 2.5ft to 4 ft in length, they get obese quickly when they are kept in improper conditions and when they are grossly overfed as adults. They are strong survivors that are sold as $5 disposable petstore lizards and end up recieving care equivalent to their purchase price, so many dont live long or grow much, etc etc. The best care sheet anywhere on them is ProExotics care sheet, as caresheets are only start point references not a guide to complete life care. The best place to start is with Daniel Bennetts book on Savannah monitors. 1 mouse a week is not enough if kept in proper conditions they burn that off in 24-48 hours. The canned monitor food does nothing really for them, but whole animal foods are the best (rodents, roaches, crickets, birds, etc) by far for them. They need a high surface basking temp (130 f), yet a temp gradient that goes from 86f-68 if possible to make use of their food etc. They need a digable substrate (dirt) to burrow in and conserve moisture, they need fresh drinking water yet if they are soaking in it its a message that changes are needed in temps etc. A good description of the animals cage in detail would be needed to go any farther.